[c++-pthreads] concrete library-code example

[Jason Merrill]

On Wed, 24 Dec 2003 22:42:19 -0800, Nathan Myers <ncm at cantrip.org> wrote:

> On Wed, Dec 24, 2003 at 11:55:59PM -0500, Jason Merrill wrote:

>> This is not cancellation-safe C under the current POSIX standard, if
>> c_function_or_system_call is a cancellation point.  If it is, cleanups
>> are run and the thread is terminated.  To be truly thread-safe, the
>> user must use pthread_cleanup_push/pop so that the resources are
>> released on  cancellation.
>
> I think that is what I said.  It was thread-safe code until the 
> semantics of calls it uses changed without changing the names.  
> What was good code is now broken, in a POSIX C thread-cancellation 
> environment.  

In what sense was it ever thread-safe?  It was fine for non-threaded code,
but has always been broken in a POSIX threads environment.

>> Implementing cancellation by throwing an exception has no effect on
>> the thread-safety of this code.
>
> In other words, because (under the C binding) this code has already 
> been broken, throwing exceptions through it won't break it any further.

Yes.

> However, we're not talking about the C binding here.  This list is for 
> discussion of a C++ binding, which need not be so procrustean as the
> C binding.

Currently, the C++ binding is the C binding, so your example is broken
under C++ as well.  And it would seem rather odd for the C++ binding to go
to great lengths to accommodate code which is broken under C, when C++
already provides a simple way to express cleanups.  C++ code doesn't need
to mess with pthread_cleanup_push; it can just use destructors.

>> > Jason, do you not consider those libraries worth preserving?
>> 
>> No, they're already broken.
>
> You answered the question with reference to the C binding.
>
> Defining a C++ binding is the purpose of this list.  With an 
> appropriately-defined C++ binding, those libraries need not be broken 
> *when linked as part of a C++ program*.  In other words, you, as a C++ 
> implementer, have the power to choose whether to preserve or to break 
> those C and C++ libraries, regardless of what has been decided for
> the C binding.

On the contrary, these libraries are broken.  You are proposing to un-break
them.

Jason